Thermostat - differential setting

Discussion in 'General Discussions' started by brandoree, Feb 10, 2009.

  1. brandoree

    brandoree Member

    This probably applies to most programmable thermostats.

    What advantage does one gain or lose by changing the temperature differential setting (the amount of degrees the thermostat has to sense before a call for heating or cooling?

    Mine is set to the default which is 0.5 degrees. However, it is adjustable up to 4 degrees.

    I realize that increasing this setting would have a direct effect on temperature swings and in effect, the comfort level. But why would one want to do this?

    Does increasing the time between calls (by increasing the differential number) create some sort of savings?

    I have a 2 stage Waterfurnace Envision with an ECM blower. Does that matter?

    I just don't get why prolonging the inevitable (calling for heat or cooling) has any advantage.
  2. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I do not

    think it would save much if anything. Heat pumps and hydronics system are "slower" to respond than fuel fired forced air systems. The ability to change the differential between first and second stage and then aux. heat could have advantages.
  3. TechGromit

    TechGromit Member

    This all goes back to weather turning your thermostat down at night saves you money. The traditional view point has been that turning down your thermostat down at night doesn't give you the same savings benifits of a Gas or Oil system, because Geo systems operate at higher efficencies and have a longer recovery time. For Gas or Oil systems, it's been proven by the department of energy so long as the setback is 8 hours at night or 10 hours during the day, setback save energy, even when you consider the furnace runs longer to get the temperature back up to normal levels.

    Another forum set out to scientificially answer this age old question, rePosted here:

    Qouted from a0128958: Does a Tstat Set-Back/Recovery Scheme Really Save Money with GSHP Units? > GreenBuildingTalk - Green Building Forums on Insulating Concrete Forms (ICF), Structural Insulated Panels (SIP), Radiant Heating, Geothermal Heat Pumps, Solar Power > GreenBui

    Here is my conclusion to the experiment, designed to answer the question, does a tstat setback-recovery scheme really save money with GSHP units.

    First 5 days: Tstats set to constant 68° on a 7x24 basis. Result was avg index value of 1.4 KWH/HDD, and each individual's day index was within 10% of the avg. Daily results were:

    Sun 22.7 KWH 17.5 HDD 1.3 Index 48° avg outside temp 173 KBTU consumed Cloudy
    Mon 31.9 KWH 22.8 HDD 1.4 Index 42° avg outside temp 263 KBTU consumed Cloudy
    Tue 44.6 KWH 29.4 HDD 1.5 Index 36° avg outside temp 373 KBTU consumed Cloudy
    Wed 44.4 KWH 32.5 HDD 1.4 Index 33° avg outside temp 376 KBTU consumed Bright sunshine

    Thu 37.9 KWH 25.9 HDD 1.5 Index 39° avg outside temp 311 KBTU consumed Bright sunshine

    At the beginning (12:01 AM) of Day 6 (Fri.) tstats were set to a 3° setback-recovery mode. The tstats were set to a constant 68° during the day, and set back to 65° at night (approximately 10 PM - 6 AM, about 8 hrs.)

    Day 6 was used as a transistion day for the structure to acclimate.

    Next 5 days using a 3° setback. Result was avg index value of 1.0 KWH/HDD. Each individual's day index was not within 10% of the avg., though. Daily results were:

    Sat 17.9 KWH 13.5 HDD 1.3 Index 52° avg outside temp 140 KBTU consumed Hazy

    Sun 9.3 KWH 10.5 HDD 0.9 Index 56° avg outside temp 78 KBTU consumed Bright sunshine

    Mon 12.2 KWH 17.0 HDD 0.7 Index 48° avg outside temp 95 KBTU consumed Bright sunshine

    Tue 17.6 KWH 15.8 HDD 1.1 Index 49° avg outside temp 140 KBTU consumed Bright sunshine

    Wed 23.3 KWH 21.0 HDD 1.1 Index 44° avg outside temp 197 KBTU consumed Bright sunshine


    1. The index value dropped from 1.4 to 1.0 KWH/HDD. Thus, I think it's at least reasonable to conclude that a setback-recovery scheme does not incur increased KWH usage (cost). In other words, feel free to set the tstat cooler at night.

    2. If you accept short, 5 day experiment time periods, and if you accept the many data point graph I published earlier showing a linear relationship between KWH and HDD, then it's probably reasonable to conclude there's actually a cost savings associated with a tstat setback-recovery scheme, within reason.


    1. There are many variables that affect the linearity of a structure's KWH consumption vs. HDD. The variables I observed included solar gain of the structure, wind, eveness of the weather pattern across each 24 hour period, and amount of internally generated heat (i.e., cooking, lighting).

    2. Using many more than 5 or 10 data points, it's clear my home has a linear relationship between KWH and HDD, for outside temps of 60° and colder (see previously published graph). I believe this is because it has enough air leakage to average out influencing variables.

    3. My guess is my home's air leakage is average. And thus the above conclusions may be reasonably applicable to many homes, noting the next comment on capacity.

    4. The above experiment results were obtained operating the residence at the lower end of heating capacity. As a result, no Aux. heat, or even 2nd stage heat, is ever required for my home. And thus my EWT stays in a range of 2 - 8° below 'down deep earth temp' (68°), resulting in constant favorable efficiencies. I would have reservation concluding tstat setback-recovery is cost effective/neutral if resulting run times cover a good portion of the day (i.e. the GSHP(s) struggle to catch up each day).


    1.) My tstats are set to a 1.2° heating differential (largest possible), and set to not allow 2nd stage to turn on until 40 min. has elapsed in 1st stage.

    2.) My structure is 3400 s.f., all single story, with 2 GSHP units (WaterFurnace Envision 5 and 3 ton units, both dual stage, variable speed).

    3.) Window treatments were opened/closed at same general time points each day.

    4.) I don't have any aux heat capability.

    Best regards,


    :End Quote

    The short answer is yes, Setbacks do save energy. One has to remember that the same rules apply, that the setback must be at least 8 hours at night to reap any benifits. Also please note that AUX was not enabled during the experiement, Your results will vary by having faster recovery times with AUX heat enabled. -TechGromit
  4. Palace GeoThermal

    Palace GeoThermal Well-Known Member Industry Professional Forum Leader

    Increasing the differential will decrease the number of times the heat pump starts during a day. This will prolong the life of your heat pump.
  5. Geotech

    Geotech Member

    My experience with setting up homes with geo or air to air (which most of these homes are zoned) heat pumps if you set-back more than 4o's you will bring on the aux. heat on cold mornings it just takes to much time to recover and remember most people do not have T-stats that are capable of setting delta-T's and if they do, they do not know how and that's a good thing. My best advice is to experiment every home and system is different even if they have the same sq. footage and same units, caution! go slow you do not want any suprises at the end of the month. GOOD LUCK!!!:)
  6. brandoree

    brandoree Member

    My thermostat has a "smart recovery" feature that will slowly bring up the temp after being set back by bringing it up slowly, trying to keep it in stage 1 the whole time.

    I have not tried it since I keep my thermostat at 69F all the time.

    I have found that if I move my thermostat up only 1 degree, the aux heat kicks in!

    I swear that it never used to do that - - used to be about 2-3 degrees.

    I'm not sure if the thermometer is a "smart" thermometer and has a "learning" mode to it of some sorts.
  7. TechGromit

    TechGromit Member

    I disabled mine, I don't want the AUX heat to kick on... ever. Unless the system is broken, I don't want expensive AUX heat to run. Thus far even with the cold tempertures cold mister has thrown at us this winter the system is able to keep the house comfortable. Recovery time without AUX heat is less than an hour with a 5 degree setback.
  8. Looby

    Looby Member Forum Leader

    Same problem here. My thermostat is as "smart" as a box of rocks.
    It's a Waterfurnace TP32W02 (re-branded White-Rodgers 1F95-1271).

    What make & model t-stat do you have? I'm about to switch to a
    Honeywell VisionPro IAQ (YTH9421) with the outdoor thermometer
    option. It allows you to lock-out aux heat when outside temperature
    is above a user-selected threshold.

    IMO there's a "buisness opportunity" for a t-stat specifically designed
    for residential geo heat pumps. One-size-fits-all "universal" t-stats
    aren't really a good match for gradual-recovery geothermal systems.

    Unlike an air-source pump, I never want to lock-out the compressor,
    and I never want to call for aux heat when the leaving air temp is
    above <user-settable threshold>, or when the room temperature is
    increasing at a reasonable rate.

  9. brandoree

    brandoree Member

    My thermostat is a Waterfurnace TP32U03.
    I don't see much difference in most of them

    And I have also turned my Aux heat off via the dip switches on the circuit board

    My thermostat has a "hidden" menu item - not shown on the instructions as to whether it displays when aux heat is on. Mine was off. I turned it on - before I flipped the dip switch.
  10. TechGromit

    TechGromit Member

    My Thermostat is a White Rogers too, I bypassed the thermostat and flipped the dip switch on the Water Furnace System Circuit board off. The AUX never comes on now. I haven't tried to switch to emergency heat yet to see if it still works in that mode.
  11. Looby

    Looby Member Forum Leader

    I did the same, but when the t-stat is calling for aux heat,
    the WF control logic still shuts down the desuperheater's
    circulating pump.

    I have to constantly keep an eye on the t-stat and turn
    it down a little every time it starts flashing "+2". If
    I just ignored it, I'd be getting virtually no DHW assist.

    I guess I could add an ON/OFF switch to interrupt the
    'W' signal -- but the correct solution is to throw that
    brain-dead POS in the trash.

    ...Murphy was an optimist,


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